This invention relates to aerosol surface cleaning.
Surface cleaning is an important step in making, e.g., semiconductor memories, printed circuits, flat panel displays, and CD-ROMs. Foreign material on a wafer surface has a direct bearing on process yields, especially as device sizes become smaller. Even a very small fraction of a monolayer of sodium ion contamination on the surface of an integrated circuit may cause the circuit to malfunction.
The size of the smallest particle capable of causing a defect in an integrated circuit is decreasing as the semiconductor industry pushes for smaller circuit dimensions. For example, the target resolution for the next generation of semiconductor devices is 0.35 .mu.m or less, which is already beyond the removal capability of many cleaning technologies. According to a well-accepted rule, foreign material on the order of one tenth of the size of the target resolution should be removed to make a fabrication process commercially viable. Accordingly, surface cleaning processes should remove foreign material on the order of 0.03 .mu.m in size from the surface of next generation's semiconductor wafers.
In semiconductor cleaning technology, most surface cleaning systems use chemicals in large volumes of purified deionized water that are selected to remove thin organic or oxide films and metal residues. Typically, these cleaning systems require immersing a silicon wafer in a prescribed sequence of one or more baths of liquid cleaning solutions on a wet bench. To reduce the consumption of these expensive solutions, which are also difficult and expensive to dispose of, a number of alternative liquid-based schemes have been devised. For example, in spray cleaning, the liquid cleaning solutions are dispensed as a spray over the surface of a wafer. More recently, a continuous flow liquid cleaning technique has been developed that combines the efficiency of wet bench cleaning and the low consumption features of spray cleaning. Other liquid-based technologies include scrubbing with special rotary brushes, ultrasonic and megasonic cleaners, and liquid jets. Recontamination resulting from recirculation and reuse of liquid chemicals is a potential problem that can only be solved by continuous regeneration on replacement of the cleaning solutions, which considerably increases the costs of the surface cleaning process.
Wafer cleaning chemistry has remained essentially unchanged over the past 25 years, the most prevalent method in the industry still being a hydrogen peroxide-based wet-chemical process. One of the most common cleaning methods in the semiconductor industry is the RCA Standard Clean, in which wafers are immersed in several chemicals sequentially to remove particles, metallic contamination, organic contamination and native oxides.
Since cleaning represents 30% of all the process steps of the fabrication process and very large amounts of water (.about.1000 gallons/wafer) and chemicals are consumed. After the final rinse, the substrate must be dried, and to avoid "water marks", isopropyl alcohol is often used as a drying agent, which again poses an environmental hazard. The process is therefore expensive and increasingly so as wafer sizes increase, and has a considerable and increasingly worrisome negative environmental impact.
Consequently, there has been a significant desire to move toward gas-phase dry-cleaning processes, such as aerosol surface cleaning, and away from current liquid-based cleaning technologies which require large volumes of cleaning agents (acids, bases and solvents) and larger volumes of deionized rinse water, which must be properly disposed of at a great expense.
As used herein, we intend the term "aerosol" to broadly refer to a gaseous suspension of microscopic particles of a liquid, a solid, or a mixture of solid and liquid.
Aerosols of carbon dioxide snow have been used for many years to clean surfaces. For example, Swain et al. (U.S. Pat. No. 5,125,979, which is herein incorporated by reference) describe a scheme in which liquid carbon dioxide is expanded from an orifice into a thermally insulated chamber to form small carbon dioxide particles. The small carbon dioxide particles are retained in the insulated chamber until they agglomerate into large snowflakes, at which point the snowflakes are accelerated within a nozzle by a high velocity vortex of a gas and directed against a surface to be cleaned.
Hand-held carbon dioxide snow cleaners are also known. For example, Layden et al. ("High velocity carbon dioxide snow for cleaning vacuum system surfaces," J. Vac. Sci. Technol., A 8 (5), Sep/Oct 1990, which is herein incorporated by reference) describe a carbon dioxide cleaner in which ultra pure carbon dioxide is expanded from a gas cylinder through a converging/diverging nozzle. The cleaner is implemented as a gun-shaped device with a thumb-actuated handle. J. F. Williford ("Enhanced CO.sub.2 Spray Removal of Fine Particulate Contamination by Spinning the Work under the Spray," Microcontamination '94, which is herein incorporated by reference) describes a scheme for achieving enhanced particulate removal by increasing the relative velocity between a carbon dioxide aerosol spray and a substrate to be cleaned. The substrate is rotated at 3,600-6,000 rpm in a direction counter to the direction of spray impingement.
Levi (U.S. Pat. No. 5,009,240, which is herein incorporated by reference) describes a wafer cleaning system which cleans semiconductor wafers by sandblasting them with ice particles. In this system, a stream of gas entrains frozen ice particles through an L-shaped conduit to the surface of a wafer to be cleaned. After the semiconductor wafer is sandblasted with ice, residual ice is removed by evaporation.
Aerosols of solid cryogenic argon particles have also been used to clean contamination from a substrate surface. McDermott et al. (U.S. Pat. Nos. 5,209,028 and 5,062,898, both of which are herein incorporated by reference) disclose a semiconductor wafer cleaning system in which a pressurized gaseous argon-containing stream, which is at a temperature above the liquefaction point of argon, is expanded and thereby solidified to form an aerosol of frozen argon particles. The resulting aerosol is directed at a surface of wafer to be cleaned. Preferably a nitrogen carrier gas is used to accelerate the argon particles.
As used herein, we intend the term "cryogenic" to broadly refer to a physical substance (atoms, compounds, molecules, or a mixture of one or more of these components) in which at least one of its physically separable constituent components has a liquefaction temperature of less than about 110.degree. K at atmospheric pressure (e.g., argon, hydrogen, helium, nitrogen, oxygen, air, or methane).
Other compounds have been used in the formation of particulate cleaning aerosols. For example, Ohmori et al. (U.S. Pat. No. 5,147,466, which is herein incorporated by reference) describe a surface cleaning technique which uses an aerosol of frozen particles formed from various compounds selected based on the hardness of the resulting frozen particle. The compounds from which these aerosols are formed include water, methanol, glycerin, and freon 113.
While such cleaning schemes have proven to be somewhat effective for removing particles, improved cleaning results are required before such dry processes can become commercially successful. In particular, a replacement cleaning technology must be able to remove other forms of surface contamination, such as photoresist, oily residues (e.g., fingerprints), and residue from previous processing steps (e.g., ion implantation). This is especially true in the semiconductor industry, where it is necessary to clean semiconductor substrates at least as well as currently used wet cleaning techniques.